This invention relates to a process for measuring the bond of a coating on a substrate.
The bond of a coating on a substrate may be evaluated by gluing the free faces of the assembly obtained on both sides of the junction interface, to plane surfaces with jaw ends that are drawn in opposite directions on a tensile testing machine. The bond measurement then consists of making a direct mechanical measurement of the force necessary to break the junction and tear the coating off the substrate. However, very wide dispersions of the measured failure forces are always observed for test pieces made in a similar manner, which proves that uncertainties are introduced in the operating method, that are so large that in practice these tests are valueless. It is probable that gluing is responsible for defects in the uniformity with which the tension force is transmitted through the assembly, but it is impossible to compensate for this even when taking the greatest care with the preparation.
This patent proposes an indirect method of measuring this resistance to tearing the coating off the substrate, making use of ultrasound measurements. In its most general form, the invention relates to a process for measuring the bond of a coating on a substrate, characterized in that it consists of sending ultrasounds into the substrate and to the coating, picking up a series of echoes resulting from the ever increasing number of ultrasound reflections on the faces of the substrate with an interface between the substrate and the coating, evaluating an echo attenuation coefficient using a function to determine the decay of echo amplitudes as a function of the number of echo reflections, and deducing the said bond of a correlation function estimated in advance using calibration test pieces, using attenuation coefficients and values of coating bond obtained by mechanical tests on calibration test pieces.